When administered in conjunction with dopaminergic agents such as L-DOPA, entacapone prevents COMT from metabolizing L-DOPA into 3-methoxy-4-hydroxy-L-phenylalanine (3-OMD) in the periphery, which does not easily cross the blood brain barrier (BBB). Pharmacologically, entacapone is somewhat similar to carbidopa or benserazide, in that it is an inhibitor of an enzyme that converts L-DOPA into a compound that cannot cross the blood brain barrier. Carbidopa and benserazide inhibit aromatic L-amino acid decarboxylase, which converts L-DOPA into dopamine, which cannot cross the blood brain barrier.

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The most frequent undesirable effects caused by entacapone relate to the increased effects of L-DOPA, such as involuntary movements (dyskinesias). These occur most frequently at the beginning of entacapone treatment. Others common side effects are gastrointestinal problems, including nausea and abdominal pains. Diarrhea is a frequently reported and troublesome side effect that can result in unnecessary investigation, but resolves quickly on withdrawal of the drug.[1] Entacapone may cause urine to turn reddish-brown. This is a harmless side effect and is not a cause for concern. In studies with entacapone, some people have reported experiencing a dry mouth.